Conventionally, it is always necessary to repeat a photolithographic step five to six times when manufacturing a TFT array substrate and a manufacturing step is prolonged. In particular, an exposing step having a high production equipment cost is required to be used very often so that the cost of the TFT array substrate to be manufactured is necessarily increased. For this reason, a decrease in the photolithographic step can produce a great advantage of cost reduction in order to manufacture the TFT array substrate.
The photolithographic step will be described with reference to FIG. 20.
A film to be etched 2 comprising a conductor, a semiconductor, an insulator or the like is formed on a substrate 1. The photolithographic step is carried out to remove an unnecessary portion of the film to be etched 2, thereby forming a predetermined pattern. As shown in FIG. 20(a), first of all, the film to be etched 2 is covered with a resist 3. As shown in FIG. 20(b), next, a photomask 4 is used to illuminate light L, thereby carrying out exposure. The photomask 4 has a shielding portion A for shielding the exposure light and a transmitting portion C for transmitting the exposure light. When development is carried out after the exposure, the resist 3 in a portion illuminated by the exposure light is lost and the resist 3 in a portion which is not illuminated by the exposure light remains on the film to be etched 2.
Subsequently, when the film to be etched 2 is removed by etching, the film to be etched 2 in the portion in which the resist 3 remains is left without being subject to the etching (FIG. 20(c)). When the resist 3 is finally removed, the etched film 2 having a predetermined pattern is obtained (FIG. 20(d)).
The photomask 4 to be used in conventional photolithography is made binary into a portion through which almost 100% of exposure light is transmitted (the transmitting portion C) and a portion through which almost 0% of exposure light is transmitted (the shielding portion A for shielding the exposure light). When the exposure and the development are carried out by using the binary photomask, a portion in which the resist 3 is present and a portion in which the resist 3 is not present are obtained as shown in FIG. 20(b). Also in the case in which the amount of exposure light is adjusted depending on a position on the substrate to form a portion in which the resist is thick and a portion in which the resist is thin, the photomask 4 itself is made binary.
Since the etching is carried out by using the binary resist 3 as a mask, the film to be etched 2 can be patterned to have only one kind of pattern at one photolithographic step (FIG. 20(d)). Accordingly, in the case in which two or more layers of the films to be etched are provided and each layer is to be patterned to have a separate pattern, the photolithographic step is required to be carried out the same number of times as the number of the layers of the film to be etched.
If the film to be etched having two layers can be patterned so as to have each separate pattern by one-time exposure and development in place of the usual patterning of one layer of a film to be etched at one photolithographic step, that is, one-time exposure and development, one photolithographic step can be reduced. A method to implement the reduction has been disclosed in the specification of Japanese Unexamined Patent Publication No. Hei 10-163174. In addition to the portion (transmitting portion) through which almost 100% of exposure light is transmitted and the portion (shielding portion) through which almost 0% of exposure light is transmitted, the photomask is provided with a portion in which the amount of transmission of the exposure light has an intermediate value therebetween. Consequently, the resist is developed to have two different thicknesses and is removed by ashing, and at the same time, the film to be etched is subjected to dry etching. Thus, two films to be etched which are formed of different materials or having different thicknesses are patterned by carrying out etching at one time, thereby reducing the number of photolithographic steps.
In the following description, a portion on a photomask through which the partial exposure light is transmitted other than the portion (transmitting portion) through which almost 100% of exposed light is transmitted and the portion (shielding portion) in which 0% of exposed light is transmitted will be referred to as “a halftone portion (or a partially transparent portion)”, exposure using the halftone portion will be referred to as “halftone exposure” and a portion of the resist which is subjected to the halftone exposure will be referred to as “a half tone resist”.